1. Field of Invention
The invention relates to a method for computer assisted localization of the distal locking holes of an implanted intramedullary nail and to a device for the computer assisted distal locking of an intramedullary nail.
2. Description of Related Art
It is known that one of the most difficult steps of intramedullary nailing of e.g. femoral shaft fractures is distal locking—the insertion of distal interlocking screws, for which it is necessary to know the positions of the distal locking holes (denoted in the following as DLHs) of the intramedullary nail (IMN). The process of locating and inserting the distal interlocking screws is complicated by the nail deformation during insertion. It is known that deformation occurs in several planes due to medio-lateral (ML) and antero-posterior (AP) flexion of the intramedullary nail during its insertion. The reason for the wide variations of the insertion-related intramedullary nail deformation is due to the fact that the nail has to deform to the shape of the medullary canal upon insertion. The shape of the canal varies widely from person to person. It is not possible to predict how the nail will deform accordingly. In a conventional surgical procedure the surgeon depends on intra-operative X-ray means for providing precise positions of the DLH's. It requires positioning the axis of the fluoroscope coaxial to the locking holes so that these holes appear perfectly circular in the images. This is achieved through a trial-and-error method and requires long time X-ray exposure for both the surgeon and patient. It has been reported that the surgeon's direct exposure to radiation for each conventional surgical procedure was 3-30 min, of which 31%-51% was used for distal locking.
The object to target accurately the DLHs with as little as possible X-ray exposure has led to various attempts to develop image-based methods for recovering the positions of the DLHs.
A computer assisted intramedullary rod surgery system is known from WO 03/043485 KIENZLE. In order to determine the position of the nail axis and of the distal locking holes of the intramedullary nail two fluoroscopic images are to be acquired, one in an approximate antero-posterior direction of the bone and a second in a lateral-medial direction of the bone. Subsequently, image processing algorithms are used to calculate the position of the nail axis as well as of the distal locking holes. This known method shows the drawback that the acquisition of the mentioned two fluoroscopic images requires a repeated and cumbersome displacing and rearranging of other surgical apparatus, devices and instruments in order to rotate e.g. the C-arm of the X-ray device from the antero-posterior direction to the lateral-medial direction.
Further, a method for robot-assisted guide positioning for distal locking of intramedullary nails is disclosed in: Ziv Yaniv, Leo Joskowicz, IEEE TRANSACTIONS ON MEDICAL IMAGING, Vol. 24, No. 5 May 2005. This known method requires only one fluoroscopic image, but in a view exactly parallel to the axes of the distal locking holes, such that the distal locking holes appear as circles (and not as ellipses) in the image. In order to correctly adjust the fluoroscope an X-ray technician must use a try-and-move method several times to achieve the correct adjustment of the C-arm of the X-ray device which typically requires 2-6 images to be taken with the X-ray device. The requirement of a fluoroscopic image in a view exactly parallel to the axes of the distal locking holes therefore shows the drawback of a very time-consuming adjustment procedure of the fluoroscope.
On this point the invention intends to provide remedial measures. The invention is based on the objective of providing a method for solving the above problems using one single calibrated fluoroscopic image taken in a lateral-medial direction of the bone only independent of the orientation of the distal locking holes.